1. Field of the Invention
The present invention relates to a method for forming a semiconductor device, and more particularly to a method for forming a semiconductor device by self-aligned.
2. Description of the Prior Art
Carriers injected in a silicon-oxide-nitride-oxide-silicon device, SONOS device, or a metal-oxide-nitride-oxide-silicon device, MONOS device, can not be spread out due to the silicon nitride layer, SiN layer, locally trapping character. However, the locally trapping character is very sensitive to high temperature, the lateral charge movement is easy to happen to degrade data retention at a high temperature.
As shown in FIG. 1A, a semiconductor device includes a substrate 100, an oxide layer 102a, a nitride layer 102b, another oxide layer 102c, a plurality of implanted regions 108, i.e. drains and sources, and a gate layer 112. Carriers 103 are stored in the nitride layer 102b. Because the locally trapping character is very sensitive to high temperature, carriers 103 may be move to the region 103′ which includes none of carriers 103 at the high temperature to lose data.
The carriers 103 prevent the different data from the mixing to each other, the channel length between two bits, i.e. two implanted regions 108, which cannot be too short to keep more space between carriers 103. The semiconductor device in the prior art includes another disadvantage. The position storing electrons inside the nitride layer 102b which is away from the implanted regions 108 than the position storing holes is. The different distance between the implanted regions 108 and the position storing electrons, and holes may increase the Vth for erasing data of cells after cycling.
According to the above description, it is necessary to develop a method to form semiconductor devices to overcome the disadvantages of the prior semiconductor devices.